linux-kernel - Re: [PATCH 1/4] perf: Add 'flags' parameter to pmu txn interfaces

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Date:	Wed, 25 Mar 2015 21:00:24 -0700
From:	Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
To:	Peter Zijlstra <peterz@...radead.org>
Cc:	Michael Ellerman <mpe@...erman.id.au>,
	Paul Mackerras <paulus@...ba.org>, dev@...yps.com,
	linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [PATCH 1/4] perf: Add 'flags' parameter to pmu txn interfaces

Peter Zijlstra [peterz@...radead.org] wrote:
| On Wed, Mar 04, 2015 at 12:35:05AM -0800, Sukadev Bhattiprolu wrote:
| > In addition to using the transaction interface to schedule events
| > on a PMU, we will use it to also read a group of counters at once.
| > Accordingly, add a flags parameter to the transaction interfaces.
| > The flags indicate wheether the transaction is to add events to
| > the PMU (PERF_PMU_TXN_ADD) or to read the events PERF_PMU_TXN_READ.
| > 
| > Based on input from Peter Zijlstra.
| > 
| > Signed-off-by: Sukadev Bhattiprolu <sukadev@...ux.vnet.ibm.com>
| > ---
| >  arch/powerpc/perf/core-book3s.c  | 15 ++++++++++++---
| >  arch/x86/kernel/cpu/perf_event.c | 15 ++++++++++++---
| >  include/linux/perf_event.h       | 14 +++++++++++---
| >  kernel/events/core.c             | 26 +++++++++++++++-----------
| >  4 files changed, 50 insertions(+), 20 deletions(-)
| 
| s390 and sparc also implement the txn.

Yes, I have fixed that now. Was mostly exploring the basic txn interface.
| 
| # git grep "\.start_txn"
| arch/powerpc/perf/core-book3s.c:        .start_txn      = power_pmu_start_txn,
| arch/s390/kernel/perf_cpum_cf.c:        .start_txn    = cpumf_pmu_start_txn,
| arch/sparc/kernel/perf_event.c: .start_txn      = sparc_pmu_start_txn,
| arch/x86/kernel/cpu/perf_event.c:       .start_txn              = x86_pmu_start_txn,
| 
| Also; you add the flag to all 3 calls; does it make sense to only pass
| it to the first and save the txn type in the txn state itself? We could
| add PERF_EVENT_TXN_READ for this..

We could do that. The one small downside I see with passing the txn flag
only ot ->start_txn() is that checks like this become more complicated,
even in PMUs that don't care about the TXN_READ transactions.

@@ -1619,8 +1622,11 @@ static void x86_pmu_start_txn(struct pmu *pmu)
  * Clear the flag and pmu::enable() will perform the
  * schedulability test.
  */
-static void x86_pmu_cancel_txn(struct pmu *pmu)
+static void x86_pmu_cancel_txn(struct pmu *pmu, int flags)
 {
+       if (flags & ~PERF_PMU_TXN_ADD)
+               return;
+

The ->start_txn will need to save the transaction type in the
architecture's 'cpuhw' and check/clear in ->commit_txn() and
->clear_txn() - right ?

Sukadev

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